This invention relates to an image forming method using an electrostatic latent image and, more particularly, to an image forming method in which transparent toner, through which exposing light can pass, is used.
Recently, a compact copier has been developed and development is proceeding briskly. In a series of image forming means in which a photoreceptor with a small circumference is used, a compact image recording apparatus is proposed in which the residual toner is electrically removed from the photoreceptor in the developing area. This type of image forming apparatus is disclosed in Japanese Patent Publication Open to Public Inspection No. 133179/1988, 133180/1988, and 133171/1988.
However, problems stemming from residual toner on the photoreceptor can be found, such as uneven charging in the charging process, light decay in the exposure process, and ghosting, and color muddiness in the image forming process. Because the circumference of the photoreceptor is shorter than the length of the toner image, a portion of the photoreceptor's circumference Is used twice to form a toner image and some portions of a toner image are periodically printed. For that reason, part of the toner is left on the photoreceptor without being cleaned off and the residual toner particles on the circumference of the photoreceptor obstruct image formation.
Especially when a coherent laser beam is used to expose an image, a defective image tends to appear in the portion shaded by opaque residual toner. Especially when the residual toner is scattered in spots, image quality is remarkably damaged.
Photoconductive semiconductors which have been used in conventional electrophotography are; inorganic photoconductive materials such as selenium, zinc oxide, cadmium sulfide, and cadmium selenide; photoconductive pigments such as phthalocyanine, copper phthalocyanine, cobalt phthalocyanine, and nickel phthalocyanine; and organic photoconductive materials such as poly-N-vinyl carbazole, anthracene, and triallyl amine derivative. These compounds are dispersed in the binder resin to form a photoreceptor. In order to rapidly make a plurality of copies in a short time, a highly sensitive photoreceptor which has very rapid light decay characteristics was developed. For that reason, a photoreceptor has been used which has high sensitivity in the low magnitude of light exposure region, wherein its light decay is sensitive even in low magnitude of light exposure and its light decay curve sharply declines in the low illuminance region.
It is known that there are two methods of forming a latent image on a photoreceptor by image exposure. One is the positive exposure system in which the non-image area is exposed to erase electrical charge on the photoreceptor and a latent image is formed. The other is the negative exposure system in which the image area is exposed to erase electric charge on the photoreceptor.
The digital image forming method is conveniently used to convert images freely, adjust images, and send images by facsimile. The light distribution of the spot of light which is used in dot exposure of the digital image forming method, is similar to a normal distribution curve in which the peak is the center of the spot, even though the spot of light is modulated by the method of pulse-duration modulation. The light energy distribution curve has long gentle slopes on both sides with wave patterns of light intensity caused by diffraction. In the case of the positive exposure system, these wave patterns are so remarkable that they can be the cause of photographic fog. That is the reason why the negative exposure system is suitable for dot exposure. However, it is necessary for the negative exposure system to be combined with reversal development and toner must be prepared which can be electrostatically charged with the same polarity as the photoreceptor. When dot exposure is used, the photoconductive semiconductor of the a photoreceptor must have a light decay curve which is suitable for the above-described spot of light's distribution curve.
Accordingly, when low magnitude of exposure, in the gentle slope portion of the spot of light's distribution curve, wherein the spot of light is for erasing electrostatic charge of the photoreceptor, is picked up by the electrostatic charge erasing action in the region of low magnitude of exposure, the electric potential distribution curve of the formed latent image of a spot becomes funnel-shaped, with a broadly extended edge. Therefore, the peripheral portion of the image developed by the toner becomes blurred and fog also occurs.
Accordingly a photoreceptor must be designed to meet the requirements to form a clear peripheral image.
When the subsequent image is developed, the potential of the photoreceptor surface is not reduced to the non-development level because of absorption of light by the deposited toner on the photoreceptor which was not transferred to a transfer paper when the previous image was developed. Image density is decreased at the portion where the potential is not reduced to the non-development level.